We may have crossed wires 
as I did not mention bringing the newly-created Personal Message back to its original topic? But rather, using it to inform members what happened in the troll example with a single reply, or, to permit two members (occasionally more) to continue their back-and-forth without derailing a thread.
But it will generate notifications with each new reply made, and they will contain the full text of replies sent immediately to the email inbox of all Staff, meaning the personal space I gave people to have their discussion is not private. Unless I am misunderstanding something, which is possible.
I would anticipate members feeling rebuked by the fact any discussion made private is being monitored by Staff. It stops being a service to give people ability to hash out a topic between themselves, and becomes a kind of naughty-step/panopticon crossover.
Perhaps this is a difference of how we conceive this role, and reporting: I consider anything that manually limits (or increases) member access to any function, and anything that consitutes a caution or warning, to be worthy of logging in the Staff section for all Staff to see; but, making things PM in the examples I gave above is slightly more like moving a topic to a better category, it IS a moderator action but it is not something all moderators need to receive automatic notification of, and be plugged into until they opt out.
And where the thread is moved to a PM and someone who took past has been TL-demoted, TL-locked, suspended, silenced, or banned as a result of their actions, I would log that in Staff so that all Staff have a single easily-searchable location for 1. the username of the banned person, plus their IP where relevant and 2. any data or comments I wish to add without sharing that with other members.
Everything this would do is already covered by the ability to easily invite Staff which is there for all TL1+, and by the facility to have a Staff section to log moderator actions (or to be specific, the reason and background for these, since the action itself appears in Logs).
This is why I suggested making adding all Staff to the newly-created PM optional for forums who do not use a Staff section, or whose moderators don’t engage in this kind of reporting.